1. Field of the Invention
The present invention relates to an optical scanning apparatus, and also to an image forming apparatus comprising such an optical scanning apparatus.
2. Description of the Related Art
In recent image forming apparatuses such as a laser printer, increase of the dot density is advancing. It becomes necessary to reduce the diameter of a light beam which is to be imaged on a medium to be scanned of an optical scanning apparatus used in such an image forming apparatus. A beam in which the intensity distribution is approximated by the Gaussian distribution is often used as a scanning beam. The focal depth is drastically reduced in accordance with the reduction of the diameter of an imaging light beam. In consideration of limitation of design of flatness of an image forming plane, shape errors of components having the apparatus, temporal changes due to a temperature and vibrations, and the like, it is difficult to maintain stably and uniformly a minute diameter of the imaging light beam over the whole scan width.
Furthermore, a phenomenon occurs that the potential distribution is widened more than an imaging light beam by dispersion of charges in a photosensitive member used as a scanned plane medium. This is one of causes of the necessity of further reducing the diameter of the imaging light beam with respect to the required dot diameter.